3,676,077 United States Patent Office Patented July 11, 1972

1. 2 In the light bath process, the electrolyte contains essen 3,676,077 PURFICATION OF MAGNESUM CHILORIDE tially a major portion of lithium chloride which is of lower specific gravity than , and a minor but Norman Bell and Hung-Kei H. Lam, Walnut Creek, Calif., effective portion of the heavier which assignors to Kaiser Aluminum & Chemical Corpora 5 is the source of the metal. In addition, a very small amount, tion, Oakland, Calif. preferably about 1% by weight of the bath of an No Drawing. Filed Aug. 28, 1969, Ser. No. 853,971 or alkaline earth metal fluoride is often included to assist nt. C. C01f 5/30 in metal collection. When the naturally included sodium U.S. C. 23-296 4. Claims and impurities increase or build up in the cell 10 charge or in the cell bath, there must be added more li thium chloride to maintain the proper density relation ABSTRACT OF THE DISCLOSURE ships so that the metal collects as a lower layer in the cell. A bittern which is substantially free of sodium and po The density relationships of the individual compounds or tassium values is readily obtained by adjusting the MgCl2 substances present in such cell are shown in Table 1. concentration of the bittern to approximately 30% by 15 weight, cooling the adjusted bittern to about -10 C. TABLE I to 20° C., preferably 0° C., maintaining the bittern at Density of Magnesium and Various Cell at 750° C. and 800° C.a. such temperature until is substantially com plete, and then recovering purified magnesium chloride 750° C. 800° C. Density, LiC Density, LiC bittern. 20 Substance g-fcc. equiv.b g.fcc. equiv.b

1.5632 ------1.5500 ------BACKGROUND OF THE INVENTION 1.6671 1.77 1.6520 1.63 1,5394 0, 342 1.5102 0,120 5838 0.895 5567 0,726 In producing magnesium metal by electrolysis of a fused 1.4414 ------1.498 ------magnesium in the so-called light bath process wherein 2.7790 9.15 2.7595 8. 07 the cell charge is of lower density than the magnesium a Ganz, G.J., et al. 'Molten Salt Data Electrical Conductance, metal produced, it is very important that the content of Density and Viscosity.' Troy, N.Y.R.P.I. (July, 1964) (U.S.A.F.O.S.R. 64-00394 U.S.D.D.C.A.D. 605,938). heavier salts, especially sodium and potassium salts, be b Weight of LiCl needed to bring .454kg. of the substance to a density kept very low, suitably not over about 0.5% by weight of 0.050 g.fcc. less than that of the metal to provide the needed settling both salts, based on total weight of the feed. These salts 30 driving force. occur as normal impurities in high magnesium chloride available from salt-producing and -making Thus, from Table I it can be seen that, in a light bath cell, works and the content thereof is generally higher than is the presence of appreciable amounts of the potassium and tolerable in the light bath process. There is a need for an sodium salts increases the requirements of LiCl, signifi inexpensive and simple way of lowering the sodium and 35 cantly. It has been recognized that a difference of at least potassium content of the brines to enable them to be con about 0.034, and advantageously of about 0.050, g./cc. verted into suitable light bath magnesium metal electro between the density of the metal and that of the electro lysis cell feedstock. lyte bath effects good separation of the metal as the bot In the operation of such process, these high density tom layer. This difference is most economically obtained salts tend to accumulate in the cell. This can increase the 40 in the absence of appreciable amounts of sodium and po density of the molten salt bath to the point where the metal tassium. as it is produced goes to the top of the electrolysis zone. According to the present invention an electrolyte and For proper operation, the cells are designed to withdraw density differential under cell operating conditions are metal product from the bottom of the electrolysis zone, provided by substantial reduction of the sodium and and furthermore, the metal is protected in that location by 45 potassium contents by a simple and efficient low temper the overlying salt layer and there is less occlusion of salt ature equilibration of the starting magnesium chloride in the metal layer. Thus, the preservation of the density . By this invention, a brine containing chlorides of relationships is of great importance, to maintain the metal magnesium, lithium, sodium and potassium salts are ad as the bottom layer. In addition, since low density lithium justed to a MgCl2 content of approximately 30% by salt also tends to accumulate in the cell, it is necessary to 50 weight. This can be accomplished by the standard pro maintain a satisfactorily high ratio of lithium to sodium cedures. The bittern or concentrated brine can be pre plus potassium content in the cell feed to provide and to pared by adding MgCl2 to a more dilute magnesium chlo maintain the needed density differential between the bath ride or brine to bring the content thereof to the and the metal. This can be achieved with brines having desired level; or by solar evaporation of the dilute brine substantial lithium contents by reducing the amount of so 55 in solar ponds, or more expeditiously in a plant by heat dium and potassium in the brine. ing the brine, for example, from about 99 C. to about The above objects and others are attained by the prac 116° C. This latter point is indicated in a practical manner tice of the present invention, as will become apparent from by the presence of a detectable amount of bischofite, the description below. MgCl2-6H2O or by determining the boiling point of the 60 solution. A small sample can be quickly tested, and SUMMARY OF THE INVENTION brines of this composition will boil at 125-126 C., at a This invention concerns a method of preparing a mag MgCl, concentration of about 30% by weight. If the nesium chloride solution for making a feed for an electro boiling point of the specimen is too high, water is added to lytic cell wherein magnesium metal is produced by elec lower the salt concentration. If the boiling point is too trolysis of a fused magnesium salt; and, more particularly, 65 low, MgCl2 is added to raise the concentration. it relates to a method for producing such magnesium chlo The bittern is then cooled to a temperature of from ride solution of satisfactory low level of sodium and po about -10 C. to about 20° C. to precipitate out sodium tassium content, especially for use in a light bath electro and potassium chlorides, and the supernatant solution is lytic cell. separated off to recover a purified MgCl2 bittern. When 3,676,077 3 4 the bittern is cooled to 0° C., the preferential precipitation cipitate out sodium chloride, has the following compo of sodium and potassium chlorides is very effective. The sition: m precipitated salts can be recovered for further processing or can be discarded. Separation can be effected by any desired method, such as, for instance, screening, decanta 5 tion, centrifuging or other. In the preparation of light bath cell feedstock, the lith ium content of some starting brines is low and in this event lithium chloride is added as desired to provide the proper concentration thereof in the bittern and at least O Water the proper ratio of lithium to the amounts of sodium and potassium which accumulate in the electrolysis cell during The brine is heated at about 40 C. until a test sample operation thereof. A typical light bath electrolyte, anhy shows a trace of bischofite, MgCl2' 6H2O. The MgCl2 drous, contains from 5% to 38% MgCl2, less than one content of the bittern so produced is 32.2% and analysis percent of an alkali or alkaline earth metal fluoride and 5 shows the following: KC10.07%, NaCl 0.24%, LiCl, 1%, the balance lithium chloride. Some natural brines con MgSO, 2.2%. tain the desired amount of lithium but where lithium The bittern is cooled to about 70 F. and decanted off chloride must be added, it can be recovered from the from any solids which deposit at this point. Sufficient solid bath dippings which are periodically or continuously re MgCl, is added to bring the content thereof in the bittern moved from the operating electrolysis cell, e.g. in order 20 to 32%. A portion of the clear supernatant liquor is then to offset the accumulation therein of sodium, potassium sent to a jacketed crystallizing tank fitted with a stirrer or both. where it is cooled to -5° C. and held at this temper The bittern containing at least 30% MgCl2 is fed to ature with constant stirring for 200 minutes, or until pre a cooling zone and cooled to a temperature of from cipitation has substantially ceased. The clear solution -10° C. to 20 C., preferably to about 0° C. In the 25 is decanted off and recovered. Three other portions of the event that concentration is effected by heating to slightly bittern are treated in exactly the same manner except below the boiling point as described above, it is desir that they are cooled to, respectively, 0 C., 5 C. and able to cool the heated bittern to about from 21 C. to 15° C. The contents of the various salts in the recovered 27 C. and then to adjust its MgCl2 concentration to at clear bitterns are shown in Table II. least 30%, advantageously 32% to 32.5%, by wt., prior 30 TABLE I to the cold equilibration at -10° C. to 20° C., as de Composition of Saturated Magnesium Chloride Brines after Low scribed herein. In an operation which is preferred because Temperature Equilibration of the ease and completeness of crystallization and re Equilibration temperature------5° C. C. 5° C. 15° C. moval of the undesired amounts of sodium and potassium Armount in weight percent: salts, the bittern is fed continuously to a cooling crystal 35 MgCl2------32.00 32.2 32.25 33.00 MgSO4. 1. 40 2.2 . 2, 20 lizer where it is simultaneously cooled and agitated to LiCl--- 0.93 0 0.94 0.95 effect good deposition and separation of the precipitated NaCl--- 0.25 0.24 0.27 0.30 crystals. KC------0.5 0. ... O 0.5 The supernatant solution is separated off from the pre cipitated crystals to recover a purified magnesium chlo 40 The clear solutions are then dried and made up into ride bittern which contains less than 0.5% of both so typical cell electrolytes for a light bath operation, the elec dium and potassium, expressed as chlorides. Such bittern trolytes having the compositions shown in Table III. is useful for conversion to anhydrous condition for use TABLE III as feedstock for an electrolytic cell wherein magnesium The Effect of Equilibration Temperature on the Composition and Densities of the Resulting Electrolytic Baths metal is made by electrolysis of fused magnesium chloride 45 and particularly where such stock is for a light bath. Equilibration temperature------5° C. C. 15° C. operation and contains a majorportion of lithium chloride. Concentration in weight percent: It is an advantage of the process of the present inven MgCl2------2.0 12. 2.0 CaF2----- 2.0 20 2.0 tion that the undesired amounts of sodium and potassium LiCl--- 65. O 63.2 58, 6. can be reduced or substantially eliminated in high mag NaCl.-- 7.5 8.1 83 nesium chloride brines by a simple process which is adapt 50 KCl------3.5 4. 9, able to a variety of plant conditions. The process can Total.------00.0 00.0 00. O be carried out in pond systems, for instance, in a region Bath density at 750 C------1.5236 1.5256 1.5302 where solar heat in the warm or hot season acts to con Density differential between bath and metal at 750 C------0.0396 0.0376 0.0330 centrate the brine by evaporation, and in the cold season 55 Bath density at 800 C------5016 15035 1507 the pond temperature is sufficiently reduced to effect the Density differential between bath and metal low temperature selective precipitation of the sodium and at 800 C------0.0484 0.0465 0.0423 potassium values. In such pond system, the purified bittern can be transferred to holding ponds, where no further In this specification and claims all parts and percen precipitation occurs upon increase of the ambient tem tages are by weight unless otherwise indicated. The re perature, and the bittern can be protected from winds 60 moval of sodium and potassium values in the present and the atmosphere by suitable layers of protective, in process to susbtantial freedom therefrom for the purposes miscible liquid such as a one-half to three-inch layer of a of the electrolytic bath is enhanced by the presence of the water-immiscible oil of a specific gravity of from 1.1 to substantial amounts of lithium salts in the brine treated. 1.2. Such layer protects from contamination and also It will be understood that the above specific description prevents further evaporation of the bittern. The oil layer 65 has been given for purposes of illustration only and that can be of coal tar, residual oil and various high boiling variations and modifications can be made therein without chlorinated organic solvents. departing from the scope of the appended claims. Having now described the invention, what is claimed is: DETAILED DESCRIPTION OF THE INVENTION 70 1. A method for removing sodium chloride and potas The following specific description is given for purposes sium chloride from a solution of magnesium chloride of illustration of some modes of carrying out the present which contains chlorides of sodium, potassium and lithium invention. comprising: A brine obtained from the Bonneville Salt Flats in (a) concentrating by evaporating the solution to a mag Utah, which has been processed in the usual way to pre 75 nesium chloride concentration of at least about 30% 3,676,077 5 6 by weight, by heating the solution to a temperature References Cited from about 99 C. to about 115 C., (b) cooling the solution to a temperature from about UNITED STATES PATENTS 21 C. to about 26 C., 3,096, 152 7/1963 Hadzeriga ------23-91 (c) readjusting the magnesium chloride concentration 5 1,215,546 2/1917 Jones ------23-298 of said solution to at least about 30% by weight, 1,593,038 7/1926 Silsbee ------23-298 (d) continuously feeding the solution through a cool 1,863,751 6/1932 Kipper ------23-298 ing zone wherein the solution temperature is lowered 1,878,586 9/1932 Kipper ------23-298 to approximately 0° C., OTHER REFERENCES (e) holding the solution at a temperature of approxi () mately 0° C. for at least about 180 minutes, and Journal of Applied Chem., pp. 316-324, 1931. (f) separating solid matter from solution. 2. A method of claim 1 wherein the solution is evapo NORMAN YUDKOFF, Primary Examiner rated to provide a magnesium chloride concentration of at S. SILVERBERG, Assistant Examiner least 32% by weight. 5 3. A method of claim 2 wherein the readajusted mag U.S. C. X.R. nesium chloride concentration is at least 32% by weight. 23-91, 304 4. A method of claim 2 wherein the solution of mag nesium chloride contains .